1. Field of the Invention
The present invention relates to a rolling-part steel for a rolling part, e.g., a rolling bearing used with automotive vehicles, agricultural machinery, construction machines and machines relating to an iron and steel industry. The present invention relates more particularly to rolling parts made of the rolling-part steel.
2. Description of the Related Art
Heretofore, when the inner and outer races of a rolling bearing, e.g., a ball bearing or spherical bearing, have been made of bearing steer (e.g., JIS, SUJ-2), a tubular blank has been cold worked into the inner or outer race by cold rolling so that the inner or outer race require no lathe turning after the cold rolling. Then the inner or outer race has been carburized-and-quenched or through-quenched.
However, it is known that a microcrack may be produced in the inner cylindrical surface layer of each of the inner and outer races of the rolling bearing which causes a crack in cold rolling or plastic working.
Thus, the present inventors have provided a rolling bearing which prevents the occurrence of the microcrack in plastic working, e.g., cold rolling or upsetting (see Japanese patent application No. 63-172030).
Japanese unexamined patent publication No. 61-257452 discloses another prior-art cold-rolled low carbon steel of a good delayed fracture characteristic having a low deformation resistance and an increased deformability both provided by means of reducing P (phosphorus) and Mn (manganese) and including Cr (chromium).
However, the prior-art bearing steel entails a problem in that the deformability in cold working of the prior-art bearing steel is low and the deformation resistance in cold working thereof is high i.e., the plastic workability of the prior art bearing steel is insufficient even when the prior-art bearing steel is spheroidized.
When the deformability in plastic working of the bearing steel is low, a working ratio of the bearing steel is low, so that a low-deformable portion of the bearing steel experiences the microcrack which in turn propagates to the crack. The occurrence of this crack causes a ring fracture of each of the inner and outer races of the rolling bearing. Thus, the surface of each of the inner and outer races of the rolling bearing has previously provided a 0.4-0.7 mm excessive thickness thereon, so that the excessive thickness which experienced the crack has been eliminated by machining after plastic working.
On the other hand, if the need for the provision of and the following elimination of such excessive thickness would be eliminated, each of the inner and outer races had to be subjected to a plastic working of a somewhat reduced degree and then to an additional lathe turning machining operation into a finished shape.
In addition, when the deformation resistance in plastic working of the bearing steel is high, a great shaping load (i.e., pressure) is required, so that a tool used in the plastic working of the bearing steel, e.g., a mandrel used in cold rolling, has been damaged early which reduced its service life. Also, the plastic working rate cannot be increased, so that the shaping time of the plastic working in long.
The present inventors have discovered that an improvement of the hardenability of a rolling-part steel and a refining of crystal grains thereof are important in order to secure a necessary rolling contact fatigue life (hereinafter, referred to as a service life or rolling life) of the rolling part.